Engineering stem cells into organs: topobiological transformations demonstrated by beak, feather, and other ectodermal organ morphogenesis

Abstract

To accomplish regenerative medicine, several critical issues in stem cell biology have to be solved, including the identification of sources, the expanding population, building them into organs, and assimilating them to the host. Although many stem cells can now differentiate along certain lineages, knowledge on how to use them to build organs lags behind. Here we focus on topobiological events that bridge this gap, for example, the regulation of number, size, axes, shape, arrangement, and architecture during organogenesis. Rather than reviewing detail molecular pathways known to disrupt organogenesis when perturbed, we highlight conceptual questions at the topobiological level and ask how cellular and molecular mechanisms can work to explain these phenomena. The avian integument is used as the Rosetta stone because the molecular activities are linked to organ forms that are visually apparent and have functional consequences during evolution with fossil records and extant diversity. For example, we show that feather pattern formation is the equilibrium of stochastic interactions among multiple activators and inhibitors. Although morphogens and receptors are coded by the genome, the result is based on the summed physical-chemical properties on the whole cell's surface and is self-organizing. For another example, we show that developing chicken and duck beaks contain differently configured localized growth zones (LoGZs) and can modulate chicken beaks to phenocopy diverse avian beaks in nature by altering the position, number, size, and duration of LoGZs. Different organs have their unique topology and we also discuss shaping mechanisms of liver and different ways of branching morphogenesis. Multi-primordium organs (e.g., feathers, hairs, and teeth) have additional topographic specificities across the body surface, an appendage field, or within an appendage. Promises and problems in reconstitute feather/hair follicles and other organs are discussed. Finally, simple modification at the topobiological level may lead to novel morphology for natural selection at the evolution level.

Fig. 1. Levels of organ formation

From molecules to the organism, there are different levels of interaction. Each level is important and inter-dependent, but also operates with different principles.

Fig. 2. Topobiological transformation events during epithelial organ formation

A). A prototype animal with ectodermal and endodermal organs. While these epithelial organs appear diverse, they share similar morphogenesis related signaling pathways and topobiological principles. Modified from Chuong edit 1998. The Molecular Basis of Epithelial Appendage Morphogenesis. B). Types of topobiological transformation events. These events are meaningful only at the level of cell groups (epithelial sheets, mesenchymal condensations), not at the single cell level. We need to learn more about how molecular mechanisms contribute to these events.

Fig. 3. Feather types (A), variants (B), and topobiological events in development (C)

Panel A is adopted from Lucas and Stettenheim, 1972. Panel B is modified from Chuong, edit, 1998, “Molecular Basis of Epithelial Appendage Morphogenesis.” Landes Bioscience, Austin, TX. Panel C is modified from Chuong, C.-M. and Edelman, G.M. 1985. Expression of cell Adhesion molecules in embryonic induction. II. Morphogenesis of adult feathers. J Cell Biol 101:1027–1043.

Fig. 4. Pattern forming processes which regulates the number and size of multiple primordia within a field

Part of A is adopted from Jiang et al., 1994, . B is from Jiang et al., 1999.

Fig. 5. Models on feather evolution

Model 1 proposes elongated scales as the origin of the feather. Modified from Regal., 1975. Model 2 proposes that a series of novel topobiological transformation events, as evolutionary novelties, transform epidermal buds into complex feathers. Panel 1a and 2a are cross sections.

Fig. 6. Molecular shaping of the beak

A. Diverse beak shapes, and the basic design of beaks. By positioning localized growth zone in different numbers and positions, the beak can become different shapes.

Fig. 7. Evolution of the beak

Different shapes of snouts from reptiles, Mesozoic birds, and today's birds are represented.

Fig. 8. Topobiological events in liver development

Stippled region: growth zone. The distribution of the growth zone is changed from diffuse, to the outer layer of developing primordia, to selected regions of growing livers. From Suksaweang, et al., 2004, Morphogenesis of chicken liver: identification of localized growth. Developmental Biology. 266, 109–122.

Fig. 9. Topographic regional specificities

A) Regional specificity across the body surface is illustrated in different species of birds. They also fly using different modes with different wing shapes. B) Regional specificity across an appendage field is best demonstrated by the array of primary remiges on the wing. C) Intra-appendage regional specificity is best demonstrated by contour feathers on the trunk. Panel A is from Feduccia, (1999). “The Origin and Evolution of Birds.” Yale Univ. Press, New Haven, ed. 2.

Fig. 10. Epithelial and mesenchymal cell recombination to generate new organs

The four issues in stem cell biology (A-D) are highlighted, and ectodermal organ formation is used for illustration. A) Sources of stem cells can be from embryonic stem cells, adult stem cells, or somatic nucleus transplantation. Cells on the lateral columns indicate different stages during progression of stem cells. The downward arrows mean differentiation. The reverse arrows mean de-differentiation which eventually disappears, meaning cells are fully committed and their fates cannot be reversed anymore. B) Cell populations are expanded with the idea that the stem cell properties, self renewal and pluripotentiality, will not be lost or deregulated to become tumors. C) Competent epithelial stem cells and regional specific mesenchymal cells are combined in a proper environment to generate organs. If everything is set right, they can self-organize in normal morphogenesis. In tissue engineering, we need to learn these principles and the regulation of specificity. D) A single feather follicle would not be too useful if it is not connected to other parts of the body and coordinated as part of the system (Fig. 1, 10D). Ectodermal organs have to be connected with other systems via angiogenesis, myogenesis, neurogenesis to be fully integrated with the organism.